Composition Dependence of the Thermal Diffusion Factor for the H2–He System

Ghosh et al. and Mathur and Watson have reported maxima and minima in the curves obtained when the thermal diffusion factor, ${\alpha }_T$ , of H₂–⁴He mixtures is measured as a function of composition. These workers have interpreted their results as due to inelastic collisions in the thermal diffusion process but as yet no theoretical treatment has predicted these anomalies. Previous experimental data have also given no such indication. We have measured ${\alpha }_T$ in a 20‐tube trennschaukel at 215, 273, and 373°K at approximately 5% composition increments and find no evidence of the aforementioned behavior. We have fitted ${\text{1\hspace{0.17em}/\hspace{0.17em}α}}_T$ , which theory predicts to be a linear function of composition, to a straight line function by the method of least squares and obtained an average deviation of ∼ 2%–3% in all cases which is less than the estimated experimental uncertainty. The data of Mathur and Watson were analyzed in the same fashion with similar results for all but the lowest temperature. We have concluded that the data of Ghosh et al. are in error and, furthermore, that while we agree essentially with the experimental results obtained by Mathur and Watson between 200 and 375°K, we cannot accept their interpretation of the data. Theoretical calculations were performed for the third Chapman–Enskog approximation to the thermal diffusion factor, ${\mathrm{[\alpha }}_T{]}_3$ , using both the (12–6) and (exp‐6) potentials. The values obtained using the (exp‐6) potential with the Mason–Rice parameters agreed excellently with our experimental results.